One type of conventional power supply for activating a d.c. load includes an a.c. to d.c. to a.c. to d.c. converter energized by an a.c. power source. The first a.c. to d.c. conversion is performed by a full wave rectifier directly responsive to the a.c. power source and which drives a filter circuit to derive a d.c. voltage. The filtered d.c. voltage is applied to a variable pulse width chopper which is activated at a relatively high frequency compared to the frequency of the a.c. power supply; for example, the chopper is activated at a frequency of 20 Hz, while the a.c. power supply typically has a frequency of 60 Hz. The relatively high frequency output of the chopper is applied to a second full wave rectifier and filter having output terminals that are connected to a load via a suitable cable. The voltage at the output terminals of the second full wave rectifier and filter is applied as a control input to a controller for the pulse width of the chopper. By controlling the width of pulses derived from the chopper, the amplitudes of the output current and voltage of the second full wave rectifier and filter are controlled. The controller includes a pair of "remote sense input terminals" which, when connected directly to the load, cause the controller to respond to the load voltage so the voltage applied to the controller by the output terminals of the second full wave rectifier and filter has virtually no effect on the controller.
Typically, such power supplies are capable of delivering approximately 500 watts to a load, such that 100 amperes are supplied to the load at a voltage of 5 volts. For many applications, e.g., large computers having extensive semiconductor memories and a large amount of calculating circuitry, the necessary power can be supplied only by a number of parallel a.c. to d.c. to a.c. to d.c. converters. This is because modern a.c. to d.c. to a.c. to d.c. converters employ choppers utilizing bipolar transistors having limited current capacity.
In the past, several different systems including switched or chopped a.c. to d.c. to a.c. to d.c. converters have been devised. These systems are generally referred to as master-slave, constant current paralleling, and straight paralleling. Each of these prior art systems has at least one disadvantage.
In the master-slave systems, one power supply or converter is selected to sense and control the output voltage of each of the remaining supplies, which are slaved to track or follow the master supply. The master-slave systems involve relatively complex interconnections for logic signals derived from the master to all of the slave units. Although the master-slave system works well and the supplies equally share the current supplied to the load, if the converters are properly designed, the reliability of the system is only as good as the master converter. The entire system fails when the master converter fails; failures of the master may have disastrous consequences to the load.
In the constant current parallel systems, current limiting circuits in the power supplies or converters are operated in the constant current mode. Hence, if a given supply is overloaded, that supply becomes a constant current source for the load as the output of voltage of the particular supply drops. Because of this requirement, the output voltages of all supplies, except one, are adjusted to be a few millivolts higher than the deisred nominal load voltages.
The supplies having the higher output voltages are designed to become equal current sources, while the supply with the lowest output voltage becomes a voltage regulator. The number of constant current supplies is thereby dependent on the load requirements. However, the supplies are not capable of delivering exactly equal currents to the load. Because there is no actual current sharing amongst the "constant current" sources, there is unequal power dissipation in the different constant current power supplies. Thereby, the reliability of the units supplying most of the current is significantly reduced. In addition, the constant current paralleling systems generally produce inferior voltage regulation than the master-slave and straight parallel systems. In certain instances, the voltage regulation requirements for the load exceed the capability of a system employing constant current paralleling.
In the straight parallel systems, the power supplies or converters are directly connected in parallel with equal length cables from each supply output terminal to a common load point. To operate effectively, all supplies must be adjusted so that the output voltages thereof are exactly the same. It has been found, in fact, that it is virtually impossible to adjust the output voltages of all the supplies to be exactly the same over relatively wide load and thermal operating ranges. Hence, the straight paralleling systems are considered to be less desirable than either the master-slave or constant current parallel systems.
It is accordingly, an object of the present invention to provide a new and improved system for controlling a plurality of parallel converters which supply d.c. current to a d.c. load.
Another object of the invention is to provide a new and improved system for regulating the current supplied by a plurality of parallel converters to a d.c. load, whereby the d.c. current derived from each of the converters is substantially the same.
An additional object of the invention is to provide a new and improved, relatively reliable system for controlling a plurality of parallel converters, each of which supplies a d.c. current to a load, whereby the current requirements of the load can still be met if any of the converters should fail.
An additional object of the invention is to provide a new and improved system for controlling a plurality of converters which are connected to supply d.c. current in parallel to a common load, wherein all of the converters operate in the same manner to supply approximately equal currents to the load, with a high degree of voltage regulation.
A further object of the invention is to provide a new and improved system for controlling a plurality of parallel converters which supply a d.c. current to a common load, wherein the converters can have different characteristics and be connected to the load via cables having different lengths.
Still an additional object of the invention is to provide a new and improved system for controlling a plurality of parallel power converters that supply approximately equal currents to a common load, wherein different power supplies function relatively independent of each other so there is no interaction between them.
A further object of the invention is to provide a new and improved system for controlling a plurality of parallel converters which supply d.c. current to a common load, wherein current limiting and over-voltage protection operations in each individual supply are not hindered because of the system interconnections.
Still an additional object of the invention is to provide a new and improved, relatively economical and reliable system for controlling the d.c. power supplied by a plurality of converters to a common load wherein the system utilizes relatively low cost and low power components, and does not require any high power devices that are subject to stresses that easily cause failure.